1. Technical Field
The present invention relates in general to rock drill bits and, in particular, to an improved system, method, and apparatus for conformal bearings in rock drill bits.
2. Description of the Related Art
Rolling cone earth boring bits have a bit body that typically has three bit legs which extend downward from the body. A bearing pin extends inward and downward from each bit leg. A conventional rock bit bearing pin is cylindrical and rotatably receives a cone. There are several varieties of bearing systems used to support the cone. These bearing systems typically consist of a combination of radial and thrust bearings that may be either scaled and lubricated or unsealed and open to the drilling fluid. The contacting wear surfaces may consist of wear-resistant metals or non-metals such as tungsten carbide and/or diamond, and may engage through sliding and/or rolling. Open bearings may contain ports to force drilling fluid through the bearing system to lubricate and cool the wear surfaces.
The cones have teeth or compacts on their exteriors for disintegrating earth formations as the cones rotate on the bearing pins. A sealed, grease-lubricated bearing drill bit contains a lubricant reservoir in the bit body that supplies lubricant to the bearing pins. A seal prevents debris from contaminating the bearing and also blocks the lubricant from leaking to the exterior. When operated in a borehole filled with liquid, hydrostatic pressure acts on the drill bit as a result of the weight of the column of drilling fluid. Each bearing pin has a pressure compensation system that is mounted in the lubricant reservoirs in the bit body. A lubricant passage extends from the reservoir of the compensator to an exterior portion of the bearing pin. The pressure compensation system has a communication port that communicates with the hydrostatic pressure on the exterior to equalize the pressure on the exterior with lubricant pressure in the passages and clearances within the drill bit. The viscous lubricant creates hydrodynamic lift as the cone rotates on the bearing pin so that the load is partially supported by lubricant fluid film and partially by surface asperity to surface asperity contact.
A polycrystalline diamond compact (PDC) bearing is a type of open bearing system. The bearing pin and cone contain discreet PDC elements placed in a circumferential array on the radial bearing and in a planar array on the thrust bearing. The PDC elements on the cone slidingly engage the PDC elements on the bearing pin. Drilling fluid is driven through the bearing to cool and to lubricate the bearing. In this type of bearing system, load is supported almost entirely by surface asperity contact. Drill bits of this nature operate under extreme conditions. Very heavy weights are imposed on the drill bit to facilitate the cutting action, and friction causes the drill bit to generate heat. In addition, the temperatures in the well can be several hundred degrees Fahrenheit. Improvements in cutting structure have allowed drill bits to operate effectively for much longer periods of time than in the past. Engineers involved in rock bit design continually seek improvements to the bearings to avoid bearing failure before the cutting structure wears out.
In conventional bits (FIG. 1), even though the clearance 111 between the cavity 113 of the cone 15 and the bearing pin 117 is quite small, the high load imposed on the drill bit causes the axis 119 of the cone 115 to translate eccentrically relative to the axis 121 of the bearing pin 117. The clearance 111 is smaller on the lower side of the bearing pin 117 than the clearance 123 (e.g., 0.006 in) on the upper side of the bearing pin 117. At high loads, the clearance 111 between the lower side of bearing pin 117 and cone 115 is reduced to zero and surface asperity to surface asperity contact occurs. The different radii of bearing pin 117 and cone 115 cause the surface asperity to surface asperity contact to be concentrated in a small area on the lower side of bearing pin 117. The concentrated contact load creates large stress and temperature gradients that can lead to bearing failure.
There has been a variety of patented proposals to address this issue. For example, U.S. Pat. No. 4,403,812 discloses the use of an elastomeric suspension around the ball bearing race to take up bearing play. This compliant suspension allows the bearing elements to self-align. Other techniques have called for pre-wearing the bearings to increase surface contact area, and modifying the PDC element size and shape. Although each of these designs is workable, an improved solution that overcomes the limitations of the prior art would be desirable.